Molybdate and molybdate/permanganate conversion coatings on Mg-8.5Li alloy

A novel environment-friendly conversion coating for Mg-8.5Li alloy was obtained by immersing in a solution of molybdate. The concentration of ammonium molybdate and the addition of potassium permanganate were discussed in this experiment. The surface morphology of the conversion coatings was observed by scanning electron microscopy (SEM), and the chemical composition was investigated by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The corrosion resistance of Mg-8.5Li alloy and conversion coatings were investigated by means of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and weight loss measurement. The results showed that the coatings with cracked morphology were homogeneous and uniform. The conversion coatings were mainly composed of metal-oxide as detected by XPS. The results of electrochemical measurement and weight loss measurement revealed that the molybdate conversion coating had better corrosion resistance than bare alloy and chromate conversion coating, and the molybdate/permanganate conversion coating had lower corrosion current density and higher coating resistance than the molybdate conversion coating.

Surface morphology of molybdate (a, 10 g/L; b, 14 g/L; c, 16 g/L) and molybdate/permanganate (d, 14 g/L molybdate + 3.5 g/L permanganate) conversion coating.Highlights► Molybdate and molybdate/permanganate conversion coatings have been formed on Mg-8.5Li alloy. ► The molybdate conversion coating had better corrosion resistance than bare alloy and chromate conversion coating. ► The molybdate/permanganate conversion coating performed better in certain condition than the molybdate conversion coating in 3.5% NaCl solution.